Authors: Huaishu Xu*, Honglei Ji*, Tianying Deng*
TCL Electronics Holdings Limited, Setion78, Huifeng Road 4, Zhongkai New and High-tech.Industries Development Zone, Huizhou, Guangdong, China,
Abstract
The paper mainly describes the common problems of direct-type quantum dot backlights, such as color deviation, LED mura, etc. In order to solve the application problems of quantum dot backlight, this paper describes a new type of quantum dot direct-type solution design scheme. A special optical film can solve the common problems caused by the direct-type quantum dot backlight at present, and improve the picture quality of the quantum dot display. This technology outlook of quantum dot products is described. Through the improvement of quantum dot materials technology, the design scheme of quantum dot backlight can be simplified, and a variety of quantum dot backlight display technologies can be realized.
Author keywords
quantum dot；direct BLU; color deviation; LED mura; optical film; quantum dot reflector; quantum dot LED;
Quantum dot backlight technology is used in liquid crystal display, which can significantly expand the color reproduction capability of displays and show more vivid and more realistic picture quality. Because the quantum dot material is exposed to the air, it is easy to aging failure due to water vapor or oxygen. In order to solve the problem of the stability of quantum dot materials, the quantum dot film encapsulation technology is implemented at the current stage. That is, the quantum dot material is encapsulated between the upper and the lower barrier films with water and oxygen barrier properties. This technology makes quantum dot materials available used in liquid crystal display.
The quantum dot backlight can improve the color performance of liquid crystal display. When combined with direct-lit multi-zone backlight scheme which uses local dimming technology, it can improve the contrast of LCD on the basis of high-fidelity color display. So that the image quality has a layered sense, it could bring an immersive look for people. The application of QD film in direct-type backlight is based on the following scheme: a blue LED is used as the light source at the bottom of the backlight; a diffusion plate is placed on the blue LED, and a certain distance OD is maintained between the diffusion plate and the LED. The function of this cavity is to enable the light emitted by the LED to be mixed to achieve a certain degree of optical uniformity. A quantum dot film is placed on the upper surface of the diffuser plate. The light source emitted by the blue LED, is homogenized through the aircavity and diffuser. The emitted blue light excites the quantum dot particles in the middle of the quantum dot film and emits corresponding wavelength, such as green and red light. Above the quantum dot film, a prism film or a DBEF with a brightening effect is placed to improve the brightness of the backlight in the positive viewing direction. According to the color addition principle, blue light + green light + red light are mixed into a white light source that meets the specified color temperature according to a certain ratio. On the top of the quantum dot backlight, a piece of LCD panel is placed to control the luminous image, and finally an image with a color screen is presented. The structure diagram of the quantum dot direct type backlight module is shown in Fig.1.
Fig.1 structure of quantum dot direct type backlight module
a: LED bar; b: Diffuser plate; c: QD Film; d: Optical Film; e:LCD panel;
Generally, we design the blue LED light emission band in the range of 445nm ~ 455nm, the green quantum dot na-nometer particle emission band is in the 525nm ~ 540nm range, its FHWM≤26nm; The red quantum dot na-noparticle emission band is in the 620nm ~ 640nm Range, with FHWM ≤ 26nm. The emission wavelengths of blue LED and quantum dot na-noparticles are designed according to the color filter and color gamut standards in matched LCD screens to achieve the best brightness and color gamut levels. Current quantum dot displays can achieve DCI-P3 (CIE1976) coverage> 95% and BT2020 (CIE1976)> 80%.
2. Problems faced by direct-type quantum dot backlight applications
In the direct-type quantum dot backlight, blue light is used as the first light source, and the quantum dot film is excited after passing through the diffuser plate. The quantum dot material inside the quantum dot film absorbs the blue light photon energy and generates red and green light waves, respectively two type of light sources pass through the mixing system of the backlight cavity to form a white light. Since the blue light source and the red-green light source exist in different spaces, they cannot form a uniform color picture after being mixed by the backlight system. There are two common problems。
a. Blue or yellow LED mura
Since the blue light LEDs are arranged at a certain interval at the bottom of the backlight, there is a certain difference in the energy distribution of blue light above the LED and the no-lightarea. Due to the difference in the energy distribution of the lightsource, after entering the quantum dot film to generate red and green light energy. Although the white light is formed through the mixing of the backlight system, however, due to the difference in blue and red-green light energy in different areas, the blue and yellow color deviations appear in different areas of the picture. Generally, if a reflective LENS is used, a yellow light shadow will be generated directly above the blue LED (Fig. 2); if a transmissive LENS is used, a blue light shadow will be generated directly above the blue LED (Fig. 3). In addition, the
Fig. 2 LED mura with the reflective LENS
Area-A is yellow, area-B is blue
Fig. 3 LED mura with the transmissive LENS Area-C is blue, area-B is yellow
b. Color deviation in Local Dimming For the direct-type backlight design, because the LED light sources arranged at the bottom with a dot matrix type. In order to form a relatively uniform surface light source, it is necessary to increase the light emitting angle of the light source by LENS. So that the light source emitted by adjacent LEDs have a superimposed effect. . Based on the principle of this design, if the display works in the Local Dimming state, because only the LED light sources in a specific area are turned on, compared to the state which the LED light sources in all areas are on, there are two chroma for the same area. The received light sources include blue light and red-green light, of which red-green light has a greater impact, and the optical characteristics presented are different. In terms of optical parameters, it shows the difference of CIE-xy color coordinates of C1 region under different states (Fig.4). Generally, the deviation of the color coordinate x / y in these two states is> 1%.
Fig. 4 Color deviation with the local dimming BLU
(a)All the LED is on
(b)Local LED is on
3. The technical solution to solve the LED mura
Based on the direct-type quantum dot backlight, the blue or yellow light mura problem occurred in the design. The reason is that after the LED light source passes through the diffuser plate, the blue light energy entering the quantum dot film is uneven and the blue light transmission direction is greatly different. After the blue light wave enters the quantum dot film, the red and green light energy of different area which generated by exciting the quantum dot material inside the quantum dot film is different. To solve this problem, the paper proposes a technique for selective transmission of light waves. The same medium has different refractive indices for different wavelength bands in visible light waves. The shorter the wavelength, the higher the refractive index. On the contrary, the longer the wavelength, the lower the refractive index. That is, in the same medium, the refractive index of blue light is higher, and the refractive index of red light is lower. According to the law of light reflection, the reflectivity of a single-layer film is R, and the reflectivities R is related to the light wave length, the refractive index of the film material, and the film thickness. According to this principle, multiple layers of media with different refractive indices and different thicknesses can be used to design a multilayer film stack. By controlling the refractive index, the thickness and the number of stacked layers, the optical film can accurately control the reflectivities of different light waves. In the same incident direction, they have different transmittances or reflectivities; or for light of the same wavelength, they have different transmittances or reflectivities in different transmission directions.
For the quantum dot backlight display, the backlight scheme design uses blue LED as the light source, and after the diffuser plate in the backlight system, the red and green quantum dot materials in the quantum dot film are excited to generate the corresponding red and green light. Finally, it is mixed into a white light source with high color gamut performance. In this backlight system, blue light and red-green light are spatially separated. Therefore, the optical film that selectively transmits light waves proposed in this paper can be used to control the transmission or reflectance of blue and red-green light after passing through this optical film. So that the direct type quantum dot backlight can form a uniform light source distribution.
In the direct type quantum dot backlight, for the LED light source of the reflective LENS(Fig. 2), a blue light incident in the normal direction has a high transmittance, and the larger the angle of incidence, the lower the blue light transmittance(Fig.5); for the LED light source of the transmissive LENS (Fig.3), can adopt the design of blue light incident in the normal direction has a low transmittance, the larger the angle of incidence, the higher the blue light transmittance design(Fig.6);
Fig. 5 the solution for reflective LENS
This solution is to achieve uniformity of the quantum dot backlight by controlling the transmittance of light at different incident angles. In the application of the backlight module, this film needs to be placed between the LED light source and the diffuser plate (fig.7) The selective optical film product is applied to large-sized TVbacklight, because the thickness of the finished product is less than 100um. In order to achieve the best result, it needs to be composited with a diffusion plate to enhance its stiffness. This optical film needs to be attached to the lower surface of the diffuser plate. After the blue light is evenly distributed, the second diffusion through the diffuser plate is then used to excite the QD film.
4. Technical solutions for color deviation
In order to improve the brightness and the contrast of liquid crystal display, the direct-type backlight designs are usually used. Through the technology of local dimming, the brightness values of different local area are adjusted at the same time to achieve a high contrast effect on the display. However, when
the quantum dot film is applied to the direct-type backlight scheme with local dimming control, due to the spatial separation of blue light and red-green light, there is a problem of chromatic aberration for partitioned light control in different ranges. To solve this problem, this paper proposes an optical film that selectively reflect or transmit light waves. This optical film has high transmittance (> 90%) at blue light and high reflectance (> 90%) at red-green light(fig.8). The blue LED light source in the direct-type backlight can pass through this optical film without any obstacles. After the QD film is excited, the downward light waves generated by the QD film are reflected back directly with this optical film. In this way, it can improve the crosstalk which due to the light emitted by the QD film passes through the direct-type backlight cavity, and is reflected back to the upper direction to generate to other areas. Through the design of this scheme, the problem of color deviation of direct-type quantum dot backlight local dimming can be improved. Generally, the color deviation value can be controlled at Δx <0.005, Δy <0.005.
The application of this optical film for selective reflection or transmission of light waves in direct-type quantum dot backlights is generally placed under the QD film(fig.9). In order to improve its stiffness, and then can suitable for large-size TV. .One is to composite the optical film with a QD film, and this optical film is lower surface of the QD film. The other is to composite it to a diffusion plate, and this optical film is upper surface of the diffusion plate.
5. Opportunities and challenges
The root cause of the LED mura or color deviation problem of the direct-type quantum dot backlight discussed earlier is the spatial separation of the red, green, and blue light sources,resulting in inconsistent angular distributions of the blue, red and green light. Due to the current quantum dot materials, under strong blue light illumination, if there is not a good enough barrier material package, the luminous efficiency will be greatly reduced. In order to make the blue light source and the red and green light emitted by the quantum dot material not have a large space separation in the direct-type backlight, and as possible as an integrated light source. The uniform white light is formed after passing through the light mixing cavity of the direct-type backlight. The product scheme of the quantum dot reflect sheet can be adopted. That is, the area corresponding to the LED of the reflector is dug, filled with red and green quantum dot with suitable concentration, and the upper and lower sides are sealed together with a material that can barrier the water and oxygen. This type of quantum dot reflector is applied to the direct-type backlight, which can match the mini LED light source without secondary LENS design(fig.10).
Fig. 10 the quantum dot reflector
From the development trend of quantum dot materials, as the performance of quantum dot materials improves, especially the resistance to strong blue light irradiation and high temperature performance. It is possible for quantum dot materials to be packaged into LED light sources. That is, QD on chip. About the quantum dot on chip, LED light source directly emits white light after mixing the three primary colors of red, green and blue, which can avoid the problem of LED mura or color deviation.
6. Summary
The application of quantum dot materials in display can provide a wide color gamut. With the direct-type local dimming control technology, TV displays can have high color gamut and high contrast picture quality.
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Symp Digest of Tech Papers – 2021 – Xu – 28 3 Direct‐type quantum dot backlight scheme design
